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zootjeff 01-14-2013 09:54 PM

Converting a Water Heater to run on 110v
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Let me start by saying I have a 40 gallon natural gas water heater. I also have a 50 gallon 3" insulated 220v electric water heater. I was going to use the 50 gallon for a solar project, but now that I no longer have the drive to do that, I was thinking of converting the 220v water heater to a post storage tank for the gas water heater. The water would come into the 40 gallon gas first, and then go into the 50 gallon. So you can see the 50 gallon electric would only have to fight insulation losses 95% of the time. If I take a really long shower, then the temp would sag, but I will almost never use 90 gallons of water with this setup.. So, given all that, I want to just run my electric water heater off a standard outlet and not run a 220 line over there. As wired, it will only draw 1100 watts off of the 110 which is just fine for a 15 or 20 amp breaker and 1/4 the water heaters rating.. This is also Plenty of power for plan of just keeping up with insulation losses.. SO my question..

How do I tell if I just have a thermostat in my electric water heater that is a simple thermal mechanical switch? Or how do I tell if the thing actually requires 220v to properly switch?

Thanks for any insight..

paintdrying 01-14-2013 10:04 PM

Read a post about something similar to this topic. Seemed to be something not everyone felt was a good idea. I would ask why a 40 gallon wh is not more than enough?

zootjeff 01-14-2013 10:30 PM

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Nevermind, it looks like I found the spec sheet on the thermostat and it even shows a wiring diagram for 110v (see page 4).

This is exactly what I want to do!

40 gallons isn't enough because I put in a nice tile shower with dual shower heads plus kid hand shower attachment and body sprays.. Don't really use the body sprays but the whole family jumps in and we have basically 3 showers going at once so we can get out of the house quick in the morning. Plus Laundry, Dishes, etc. We run out all the time..

tylernt 01-16-2013 01:27 PM

Running the 50g at only 1.1kW after the 40g gas will give you a larger amount of water in a burst, but it will take a really long time for the 50g to recover -- during which time you will get lukewarm or cold water. Recovery might take all day. I wouldn't do it.

I would do it the other way around though, have cold going into the 50g electric first, then go out to the 40g gas. The gas will "recover" more quickly so that an hour after your showering+washing frenzy, you will have 40g worth of hot water available again. Then 23 hours later, the 50g is finally ready to go again too.

zootjeff 01-17-2013 02:01 AM

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I really don't want to put the 50 gal electric first cause it will wind up doing 90% of the work and electric water heaters cost twice as much as natural gas to operate.. I'm not really likely to come close to more than 90 gallons in an hour.. Probably not even 65.. So if I stay ahead of the recovery of the gas then I should easily be able to take advantage of the extra 50 gallons over a 24 hour period if I ever need it..

zootjeff 01-17-2013 05:05 AM

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I'm going by this overview and using standard BTU to watts and 100 degree delta T calculations:

My Gas water heater is 34000 input BTU and has a first hour recovery rating stamped on it of 28.8 Gallons per hour at 100f.
In theory the first hour delivery is 56.5 gallons based on the 70 percent mix of cold rule into a 40 gallon tank (28.5+28).

Normally the 4500W water heater has a gallons per hour recovery of 18.37 gallons per hour recovery. (53.37 first hour delivery) Since I'll run it at 1125W it will be 4.6 gallons per hour recovery or 39.59 first hour delivery.

So if you can just add this stuff up, in theory I have 39.6+56.5 or 96.1 gallons first hour delivery. I'll bet it is better than that as the 70% rule likely doesn't quite apply with the luke warm gas heated water feeding the electric water heater with the slowly colder water after the gas heater is running out, maybe it's 85%?

Basically if you start from scratch and with 220v heat 50 gallons from 40 to 140, it takes 41750 BTUs or 2.72 hours at 4500 watts.

Basically if you start from scratch and with 110v heat 50 gallons from 40 to 140, it takes 41750 BTUs or 10.8 hours at 1125 watts.

I really think I have the best approach here given that I don't plan to take twice as long of a shower as I did before..

rjniles 01-17-2013 06:01 AM

I would install the electric first in line and leave it turned off. That way it acts as a tempering tank. during anticipated heavy loads you can turn on the electric.

I would find a way to wire a 30 amp circuit to the WH so you would have the full power capability.

tylernt 01-17-2013 09:22 AM


Originally Posted by zootjeff (Post 1095448)
I really don't want to put the 50 gal electric first cause ... electric water heaters cost twice as much as natural gas to operate..

In that case I'd leave the 50g unplugged and install a recirculating water pump so the water in both tanks is kept hot purely by the gas WH.

Canucker 01-17-2013 11:04 PM

How much hot water do you actually need? I don't know how you're set up but if you don't already, you may be able to turn the temp of your gas heater up and use a mixing valve to lower the temp when it exits. It'll give you a longer draw of hot water off the same sized tank. Won't work though, if there's a huge difference in demand.

ddawg16 01-18-2013 01:28 AM

I'm thinking that your solar idea has a better chance of working.

Alan 01-18-2013 09:23 AM

I don't see how this is going to make any difference at all and quite frankly it makes no sense.

In the configuration you describe, the gas water heater is doing the heating of the water, and the electric is only storing it. Now : When the water is not being used, the electric water heater only has to keep up with heat loss to the atmosphere.

My question is : What difference in KWH will it really make whether it's wired at 110V or 220V ? You still have to make the same change in temperature, whether you do it more slowly with a single element, or more quickly with two elements.


joecaption 01-18-2013 09:33 AM

Sounds like a great place for a large tankless gas heater to me.

zootjeff 02-14-2013 06:52 PM

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Originally Posted by joecaption (Post 1096308)
Sounds like a great place for a large tankless gas heater to me.

The tankless I want costs 1500 for a unit that will avoid cold water sandwich with a buffer tank and achieve 95+ efficiency. I also have to route a new gasline to get the 200,000 BTU to the tankless unit. The setup I've described cost me 20 bucks for the used electric and another 40 bucks of hookup pipe/supplies. So solve my problem for 60 bucks, or 1500+ bucks. The tankless would be nice for space and total energy cost, but I bet I'll never make it back..

So I installed my dual water heater setup. I also put a plugged in power meter inline with the 110 going to the electric heater. I have the gas hot water heater set hotter than electric, so far this morning the Electric didn't even turn on once.. It cost 27 cents and ran for about 2.7 hours to get to 130f after I partial flush and filled it last night..

AllanJ 02-14-2013 10:23 PM

In other words a terrible time and place for a large tankless gas heater.

One thing I was thinking of is, do it as planned with the gas heater first and the electric heater second but, set things up so that when the electric heater comes on a circulator pump on a loop from final (electric heater) outlet back to the starting cold inlet (gas heater) comes on at the same time.

This will reduce the run time of the more energy-expensive electric elements although the less energy-expensive gas burner will run a little more. Also it speeds up the heating of what's in the electric heater (speeds up eliminating a huge cold water sandwich for next time) if you have a long usage session that overdraws the gas heater and a lot of cold water infiltrates the electric heater. But I don't have in mind the best way of installing this.

zootjeff 02-18-2013 11:32 AM

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Both tanks are installed and I had a full day of yard work with my family. My wife and kids went in to get showered off and I came in to shower as they were getting out. Normally I'd have about 4 minutes before the shower would get pretty cold, but I had a great relaxing hot shower right after them.

I put a plug in power meter that tallies the power usage and after my shower the electric water heater ran for a little under hour to fully recover.. Using about 10 cents of power.. I'd say that was a success..

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